In recent years, with rapid development of various Internets and multi-media applications, traffic in a communications network is growing rapidly. An access network, a metropolitan area network, and a backbone transmission network all have a growing requirement on device upgrade, so as to meet a growing network traffic requirement. An optical sending and receiving module is a core unit of an optical network. The optical sending and receiving module characterized by small form factor, low power consumption, multi-channel, and low cost becomes a development trend. As core components of the optical sending and receiving module, an optical transmitting assembly and an optical receiving assembly also must be developed to be characterized by small form factor and multi-channel. An integrated encapsulating technology may realize small form factor of a multi-channel optical assembly, that is, encapsulating a multi-channel laser chip or a detector chip into a same tube shell. In the optical assembly, besides the laser chip and the detector chip, some passive components are also required to implement a passive processing function of an optical signal such as dividing optical power into multi-channel signals, wavelength division multiplexing/de-multiplexing, polarization state combining and separating, and the like, so as to constitute a complete optical transmitting assembly function or optical receiving assembly function. The passive component may be classified into two types: one type is based on free space optics, that is, a light beam is propagated in air or other uniform mediums; and another type is based on waveguide optics, that is, a light beam is propagated in an optical waveguide. Each of the two types of passive components has advantages and disadvantages. For a small-form-factor optical assembly with more than four channels, the passive component based on a planar optical waveguide chip has an advantage over the passive component based on free space optics.
For a pigtail optical transmitting assembly and a pigtail optical receiving assembly, coupling a single-mode fiber and the passive component is one of key technologies. If the passive component uses the planar optical waveguide chip, how to reduce insertion loss between the single-mode fiber and an input/output optical waveguide is a difficulty faced by many developers. Diameter of a core region of the single-mode fiber is 9 micron, while size of a core layer of a single-mode optical waveguide is much smaller, for example, 4 micron×4 micron. A difference between speckle sizes of the single-mode fiber and the single-mode optical waveguide results in very large insertion loss, for example, reaching 2 dB. For the optical transmitting assembly or the optical receiving assembly, such insertion loss cannot be accepted.
In order to reduce the insertion loss between the fiber and the waveguide, a solution is provided so that an entity coupler of which renders a narrower (wider) linear gradient is used to reduce the insertion loss between the fiber and the waveguide. Since a manufacturing process of the coupler is planarization, an advantage of the coupler is simple design and a disadvantage is that size of a light spot is adjusted only transversely (horizontally) but cannot be adjusted vertically. Therefore, the light spot size of the single-mode waveguide cannot be made to close to the light spot size of the single-mode fiber. The coupler has a certain effect on an optical waveguide with a low refraction index difference, while the coupler has a non-obvious effect on an optical waveguide with a high refraction index difference. Therefore, the insertion loss during coupling optical waveguide signals on two sides of the coupler cannot be reduced significantly.